Caldwell has discontinued this type of balance. The Andersen box balance is a smaller box, so if you want to keep your old sashes, you will need to do some modifying. If you're interested, please tell me your sash glass size and the weight of your sash.
Double Hung: Upper 31 1/2" by 25" app 15 pounds. Lower 38 1/2" by 25" approx 20 pounds. Extra weight is due to replacing the original glass with leaded pane windows.Weight is an approximation.
Bob